Each year, more than a million women are diagnosed with breast cancer, worldwide. Our understanding of the molecular mechanisms that underlie this disease are advancing rapidly, in part due to an enormous application of resources and effort to this problem. However, our understanding of how to modify the risk of developing breast cancer lags far behind. Epidemiological studies have demonstrated that a full-term pregnancy before the age of 30 provides a life-long reduction in the risk of developing the most common form of breast cancer, luminal ER+/PR+ disease. The effect is not minor, with a drop of nearly 50% in the probability of developing disease. This is a highly conserved phenomenon, since both rats and mice are afforded a similar, or even greater protection, by pregnancy or by a course of treatment with pregnancy-associated hormones. Mammary epithelial stem cells give rise to all of the cell types in the gland. When the gland involutes after pregnancy, it is thought that the only cells that survive to give rise to the remodeled post-partum gland are the stem cells themselves. Thus, these likely preserve some epigenetic memory of the pregnancy event. We propose that this memory modifies the ability of these cells, or their estrogen-responsive progeny, to become tumorigenic in response to oncogenic insults. We will map the complete epigenetic state of the mammary stem cell genome in nulliparous mice and humans. We will then search for changes in the epigenome that occur upon pregnancy or upon treatment (in mice) with a course of pregnancy associated hormones. We will correlate these with changes in the transcriptional output of short and long RNAs. It is our hope that these comparative studies will focus us on pathways that underlie protection and that we will be enabled in the long run to devise a clinical intervention that will substitute for the beneficial protective effects of early pregnancy. PUBLIC HEALTH RELEVANCE: More than a million women each year are diagnosed with breast cancer. Numerous epidemiological studies over the past 40 years have established that an early, full-term pregnancy reduces the risk of the most common form of breast cancer by half, and an even greater reduction in risk can be produced in rodents by pregnancy or a course of hormone treatment. We propose to identify common epigenetic changes that occur upon pregnancy or hormone exposure in humans and mice to identify candidate mechanisms for breast cancer risk reduction.